The present invention relates to a substrate material for wiring, which is an intermediate material constituting a circuit board, and a substrate material for printed circuits using the substrate material for wiring.
Substrate materials for wiring such as printed circuit boards have, at one side, slots for integrated circuits and terminals for connecting various electronic parts and, at the other side, printed conductive paths for connecting the parts, and have numerous applications, such as a key member of electronic appliances.
FIG. 3 is a perspective view showing a printed circuit board. In FIG. 3, a substrate material 1 comprises a sheet-shaped material composed of an insulating material (e.g. epoxy resin or glass) and a conductive metal 2 formed in the sheet-shaped material by plating or the like so as to electrically connect the two surfaces of the sheet-shaped material. On the two surfaces of the substrate material 1 is laminated a thin film layer 3 in which a predetermined circuit is formed; above and below thin film layer 3 are formed terminals and conductive paths 4 by a thin-film method, a printing method or the like; thereby, a printed circuit board is constituted.
The substrate material 1 used in printed circuit boards has heretofore been produced, for example, by producing a sheet-shaped material (glass-epoxy substrate or the like) composed of an insulating material such as epoxy resin or glass, making through holes for electrical connection at the predetermined positions of the sheet-shaped material by drilling, filling the through holes with a conductive metal such as copper by means of plating or the like, and sealing the through holes with a sealant.
In the above production, however, drilling in the shaped material generates chips and has raised concerns about producing defective products, and plating methods yield a higher possibility of generating cracks at the periphery of the substrate material, inviting poor electrical connection.
Further, since a glass-epoxy substrate has a high thermal expansion coefficient of about 50 ppm/xc2x0 C. in a direction of thickness (vertical direction) though it has a low thermal expansion coefficient of about 15-20 ppm/xc2x0 C. in a direction of a plane (horizontal direction), which causes a mismatch of thermal expansion coefficient with the copper material provided by plating in a throughhole. Therefore, the throughholes are lacking in reliability.
Further, in the drilling of the throughholes, the marginal ratio of throughhole length (substrate thickness)/hole diameter is about 5 and hence, for example, the lower limit of hole diameter is about 0.2 mm in the case of a substrate material having a thickness of 1 mm. That is, in drilling methods, it has been difficult to highly integrate the printed circuit board by decreasing the diameter of through holes.
In view of these problems, there was proposed in JP-A-49-8759 a circuit board obtained by inserting, into a frame electric wires composed of Ni, Co or the like, pouring thereinto a molten insulating material composed of an epoxy resin or the like, curing the insulating material, and cutting the resulting material at a plane perpendicular to the metal wires to allow the two surfaces of the substrate to have electrical connection to one another.
In this circuit board, however, an insulating material such as epoxy resin causes, in curing, a volume shrinkage of about 2 to 3%, which has impaired the dimensional accuracy of throughhole pitches, etc. This has been a big drawback because strict dimensional accuracy is required in highly integrated printed circuit boards.
Further in this circuit board, since no attention is paid to the difference in thermal expansion between the circuit board and thin film layer laminated on one or both surfaces of the circuit board or metal wires, peeling may occur between the circuit board and the photoprocess layer or the metal wires owing to temperature differences during use.
Further, the circuit board does not show sufficient thermal resistance in heat treatment steps at 100xc2x0 C. or more upon forming a wiring pattern on the circuit board. Moreover, it does not show sufficient durability and/or reliability under conditions of high temperature and/or high humidity.
The present invention has been made in view of the aforementioned conventional problems, and the present invention has an object of providing a substrate material for wiring, which has high dimensional accuracy and is excellent in thermal resistance in heat treatment steps, durability and reliability under conditions of high temperature and high humidity, and can avoid peeling of a thin film layer from the substrate during use; particularly providing a substrate material for printed circuit, in which peeling of metal wires from the substrate material can be avoided and reliability of through-holes can be improved as well as the aforementioned properties.
According to the present invention, there is provided a substrate material for wiring, characterized in that said substrate material comprises an insulator composed mainly of epoxy resin and ceramics having a thermal expansion coefficient of 2 ppm/xc2x0 C. or less, and that the thermal expansion coefficient is isotropic.
The thermal expansion coefficient of the present substrate material is 5-30 ppm/xc2x0 C., and difference in thermal expansion coefficient between horizontal and vertical directions is 30% or less.
In a substrate material for wiring of the present invention, the epoxy resin preferably contains glycidyl ether type epoxy resin, and the glycidyl ether type epoxy resin is further preferably bisphenol A type epoxy resin and/or novolac type epoxy resin.
According to the present invention, there is further provided a substrate material for wiring, characterized in that said substrate material comprises an insulator formed by pouring a material composed mainly of epoxy resin and ceramics having a thermal expansion coefficient of 2 ppm/xc2x0 C. or less into a mold, and that the thermal expansion coefficient is isotropic. In the present invention, it is preferable that the viscosity of the material is 10 Paxe2x96xa1s or less at the time when the material is poured into a mold.
Further, a substrate material for wiring of the present invention has a characteristic that the thermal expansion is 10 -25 ppm/xc2x0 C.
Moreover, a substrate material for wiring of the present invention is preferably made of a molding material containing 100 parts by weight of glycidyl ether type epoxy resin as major material, 50-150 parts by weight of an acid anhydride as a hardener, and 300-3000 parts by weight of ceramics having a thermal expansion coefficient of 2 ppm/xc2x0 C. or less.
The ceramics having a thermal expansion coefficient of 2 ppm/xc2x0 C. is preferably amorphous silica, and the amorphous silica preferably has an average particle size of 50 xcexcm or less. Further, it is preferable to add, to the molding material for a substrate material for wiring of the present invention, 0.5-30 parts by weight of a coupling agent, and further 30-120 parts by weight of a flame retardant.
A coloring agent is preferably added thereto for coloring, and the coloring is preferably black.
In the substrate material for wiring of the present invention, copper plating is preferably applied to a surface or both surfaces of the substrate material for wiring, and a copper foil is preferably applied to a surface or both surfaces of the substrate material for wiring.
Furthermore, according to the present invention, there is provided a substrate material for printed circuit, characterized in that conductive metal wires are embedded at given pitches in the aforementioned substrate materials for wiring.